Learn German: Niemand kauft hier teure Zitronen. - Nobody buys expensive lemons here.

When the adjective teuer gets an ending, it drops the unaccented e before the r to make it easier to pronounce. So, teuer + -e becomes teure, not teuere. Another common adjective that drops an e when declined is dunkel (dark), which becomes dunkle.

Why does teure get the -e ending here?

This is the rule for plural nouns with no article in front of them (the "zero article"). Zitronen is the direct object here (accusative case) and is plural. Because there is no article like die (the) or meine (my) to show the case, the adjective has to do the heavy lifting by taking the strong ending -e.

Why is the verb kauft in the singular form? Isn't "nobody" talking about a group of people?

Just like "nobody" in English, the German word niemand is grammatically singular. It is treated as a third-person singular subject, exactly like er, sie, or es. That is why the verb takes the -t ending: niemand kauft.

Why does hier come before teure Zitronen? Could I put it at the end?